Optical Oxygen Sensors Show Reversible Cross-Talk and/or Degradation in the Presence of Nitrogen Dioxide

A variety of luminescent dyes including the most common indicators for optical oxygen sensors were investigated in regard to their stability and photophysical properties in the presence of nitrogen dioxide. The dyes were immobilized in polystyrene and subjected to NO2 concentrations from 40 to 5500 ppm. The majority of dyes show fast degradation of optical properties due to the reaction with NO2. The class of phosphorescent metalloporphyrins shows the highest resistance against nitrogen dioxide. Among them, palladium(II) and platinum(II) complexes of octasubstituted sulfonylated benzoporphyrins are identified as the most stable dyes with almost no decomposition in the presence of NO2. The phosphorescence of these dyes is reversibly quenched by nitrogen dioxide. Immobilized in various polymeric matrices, the sulfonylated Pt(II) benzoporphyrin demonstrates about one order of magnitude more efficient quenching by NO2 than by molecular oxygen. Our study demonstrates that virtually all commercially available and reported optical oxygen sensors are likely to show either irreversible decomposition in the presence of nitrogen dioxide or reversible luminescence quenching. They should be used with extreme caution if NO2 is present in relatively high concentrations or it may be generated from other species such as nitric oxide. As an important consequence of nearly anoxic systems, production of nitrogen dioxide or nitric oxide may be therefore erroneously interpreted as an increase in oxygen concentration.


Experimental section Materials and Methods
All chemicals were purchased from commercial suppliers. Unless otherwise indicated, chemicals were used as received. Suppliers of indicator dyes include Frontier scientific (www.frontiersci.com/), Acros Organics (now Thermo Fisher Scientific, www.thermofisher.com), Sigma Aldrich (www.sigmaaldrich.com) and Kremer Pigmente (www.kremer-pigmente.com/). Details are listed in Table S1.
SEC chromatographic analysis was performed on a WGE Dr. Bures SEC3010 (www.wge-dr-bures.de) with THF as eluent (1 mL min−1) and refractive index (RI) detection. Poly(styrene) standards were used for calibration. IR spectra were taken on an ALPHA-P FT-IR spectrometer from Bruker (www.bruker.com) with a diamond-based attenuated total reflection (ATR) module.

Dyes
The indicator dyes were either purchased from commercial suppliers or synthesized according to literature procedures, the details are summarized in Table S1.

Immobilization of dyes in polymers
All foils of polystyrene-immobilized dyes were produced in a similar fashion. First, a sensor "cocktail" was prepared containing the respective dye and the polymer dissolved in a suitable solvent with a concentration of 10% wt. of polymer in the solvent. This "cocktail" was stirred for several hours until all the componets were fully dissolved. Then ~200 µL were pipetted onto a PET support foil and knife coated with a wet film thickness of 76.2 µm. Immediately after knife coating, a Fluoropore TM Teflon filter was carefully placed on top of the still wet sensor foil so that the cocktail would soak into the filter pores. The foils were then left to dry at room temperature for several hours.  properties. The modulation frequency was tuned depending on the unquenched lifetime of the dyes as well. Table S3 gives an overview which phase fluorometer and modulation frequency and, in the case of lock-in amplifier, what LED and filters were used. PtTFPP on fumed silica in Cytop (FS-Cytop) One sensor foil was produced with PtTFPP coupled to fumed silica and dispersed in Cytop CTL-107MK.
The coupling process was conducted according to literature procedure. 19 50 mg of the modified particles were then dispersed in a "cocktail" containing 720 mg of CTL-107MK and 780 mg Purosolve 75/00.
The "cocktail" was stirred for several hours until homogenous distribution was achieved and then knifecoated on an SiOx-modified PET foil.
PtTFPP in ECTFE Glass discs with one roughened side (diameter 8 mm, hight 1 mm) were used as a transparent support instead of PET due to poorer stability of the latter at high temperatures. The glass discs also were placed on the heating block and a small portion of "cocktail" was homogenously applied to the roughened side of the hot glass disc using a heated glass pipette. Then the discs were left on the hot heating block for another 15 minutes to evaporate the solvent, after which the heating was turned off to let the discs slowly cool to room temperature.

Poly(aryl ether) polymers
The structures of the synthesized poly(aryl ethers) are shown in Figure S1, their synthesis was conducted analogously to a procedure published by Liu et al. in 2003. 20 Table S4 shows results of SEC chromatography of the synthesized poly(aryl ether) polymers, Figure S2 contains the IR spectra.

Results
An example of a typical measurement is provided in Figure S3  Additionally, absorption spectra of PS-based foils were recorded before exposure to NO2, after exposure to 180 ppm NO2 over a duration of 10 minutes and after exposure to 5500 ppm NO2 over a duration of 30 minutes. In these cases, exposure to NO2 too place in darkness so no additional photodegradation took place. The results can be seen in Figures S4-S11.
Changes in absorption spectra and luminescent properties of selected dyes (Ir(CS)2acac, PtTPTBPF, Ru(dpp)3(TMS)2 and PtOEP) dissolved in toluene upon exposure to NO2 as well as mass spectroscopic analysis of resulting products are summarized in Figures S12-S22.
The stability of materials based on PtTFPP, PtTPTBPF and Pt8SO2TPTBP in different polymers towards NO2 was determined as for the PS-based sensors, the results are depicted in Figures S23 & S24 and in Figure 4.  Figure S4. Absorption spectra of PS foils with immobilized porphyrins before exposure to NO2, after exposure to 180 ppm NO2 for 10 minutes, and after exposure to 5500 ppm NO2 for 30 minutes. The right row shows absorption spectra normalized to the most intence absorpiton band.  Figure S5. Absorption spectra of PS foils with immobilized π-extended porphyrins before exposure to NO2, after exposure to 180 ppm NO2 for 10 minutes, and after exposure to 5500 ppm NO2 for 30 minutes. The right row shows absorption spectra normalized to the most intence absorpiton band.   Figure S6. Absorption spectra of PS foils with immobilized π-extended porphyrins before exposure to NO2, after exposure to 180 ppm NO2 for 10 minutes, and after exposure to 5500 ppm NO2 for 30 minutes. The right row shows absorption spectra normalized to the most intence absorpiton band. Zr-PDP Figure S8. Absorption spectra of PS foils doped with various phosphorescent complexes before exposure to NO2, after exposure to 180 ppm NO2 for 10 minutes, and after exposure to 5500 ppm NO2 for 30 minutes. The right row shows absorption spectra normalized to the most intence absorpiton band.   Figure S9. Absorption spectra of PS foils doped with europium complexes before exposure to NO2, after exposure to 180 ppm NO2 for 10 minutes, and after exposure to 5500 ppm NO2 for 30 minutes. The right row shows absorption spectra normalized to the most intence absorpiton band.   Figure S11. Absorption spectra of PS foils doped with fluorescent dyes before exposure to NO2, after exposure to 180 ppm NO2 for 10 minutes, and after exposure to 5500 ppm NO2 for 30 minutes. The right row shows absorption spectra normalized to the most intence absorpiton band.